Noise Figure Measurements
 

I've been playing with trying to make rough noise figure measurements
on the cheap and have a couple of questions:

(1) Are there any issues with the following setup and procedure for
making relative noise figure measurements (e.g. comparing two
receivers) ?

____________ __________ ________
| | | | | |
|Uncalibrated| | Step | |Receiver| Audio Out
| Noise |----|Attenuator|------| Under |-------*----o (0 dB)
| Source | | | | Test | _|_
|____________| |__________| |________| | |
| | 2.7k
|_|
|
*----o (-3 dB)
_|_
| |
| | 6.65k
|_|
_|_
///
Procedu
(a) For receiver 1 connect a high impedance AC voltmeter to the audio
output marked "0 dB". Record voltage with noise source off.
(b) Turn noise source on and measure AC voltage at "-3 dB" output.
Adjust step attenuator to get same voltage as in step (a). Record
step attenuator setting.
(c) Repeat steps (a) and (b) for receiver 2.
(d) The difference in noise figure between the two receivers is the
same as the difference in attenuator settings recorded in (b) and (c).
For example if the attenuation for receiver 1 is 10 dB and for
receiver 2 is 12 dB, then receiver two has a noise figure which is 2
dB less than that of receiver 1.

Assuming this is OK we move on to question 2:

(2) To avoid the expense of a calibrated noise source, I wonder if the
repeatability from unit to unit of simple low noise amplifier circuits
(perhaps a MAR-6 ?) is good enough to allow one to be used as a noise
figure standard, at least for fairly rough measurements at HF and VHF.
I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
Has anyone measured the NF performance of simple MMIC amps at HF & VHF
? Or looked into noise figure repeatability ?

73,
Steve VE3SMA

Cheap calibrated noise source, 35dB Bandwith 100kHz - 500Mhz.
http://www.elecraft.com/manual/N-gen...%20rev%20C.pdf
Noise performance of Mar's MMIC's should be noted on datasheets.
I have them "somewhere", with a little luck Google finds it too.
You can measure and calibrate your own noise generator.
Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.
Remove resistor, and switch ON the unknown ENR, Z = 50 Ohm.
The only thing to do is, ATTenuate in steps until you are at first measured
output labeled zero dB.
Add simply the dB's of the ATT units, this should be the ENR dB's.
Please correct me if I am wrong.
Homebrew ATTenuator pads can be build low as 0.1dB accurate enough.

Sources of how to measure NF are radio amateurs who do EME.
Google should come up with data if you search at "low noise" "EME" etc.
PS: Google for VE3DNL Using Noise Generators to measure Noise Figure.
Greetings Bas.

"Steve Kavanagh" schreef in bericht
om...
I've been playing with trying to make rough noise figure measurements
on the cheap and have a couple of questions:

(1) Are there any issues with the following setup and procedure for
making relative noise figure measurements (e.g. comparing two
receivers) ?

____________ __________ ________
| | | | | |
|Uncalibrated| | Step | |Receiver| Audio Out
| Noise |----|Attenuator|------| Under |-------*----o (0 dB)
| Source | | | | Test | _|_
|____________| |__________| |________| | |
| | 2.7k
|_|
|
*----o (-3 dB)
_|_
| |
| | 6.65k
|_|
_|_
///
Procedu
(a) For receiver 1 connect a high impedance AC voltmeter to the audio
output marked "0 dB". Record voltage with noise source off.
(b) Turn noise source on and measure AC voltage at "-3 dB" output.
Adjust step attenuator to get same voltage as in step (a). Record
step attenuator setting.
(c) Repeat steps (a) and (b) for receiver 2.
(d) The difference in noise figure between the two receivers is the
same as the difference in attenuator settings recorded in (b) and (c).
For example if the attenuation for receiver 1 is 10 dB and for
receiver 2 is 12 dB, then receiver two has a noise figure which is 2
dB less than that of receiver 1.

Assuming this is OK we move on to question 2:

(2) To avoid the expense of a calibrated noise source, I wonder if the
repeatability from unit to unit of simple low noise amplifier circuits
(perhaps a MAR-6 ?) is good enough to allow one to be used as a noise
figure standard, at least for fairly rough measurements at HF and VHF.
I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
Has anyone measured the NF performance of simple MMIC amps at HF & VHF
? Or looked into noise figure repeatability ?

73,
Steve VE3SMA

Steve,

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

An anecdotal story for you...

About four years ago (subsequently laid off :-( ) I was working for a
company that was making a virtual (that is, used digital signal processing)
RF/Microwave measurement system. One of our customers was a Major
communications satellite builder. They were complaining that our system was
not working right for measuring noise figure. I knew it was, of course,
since I had written the software and thoroughly tested it :-). I used two
different manual methods, one of which is exactly what you are doing (except
for using acalibrated noise source) and got the same answer as my software.
Turns out that they (the Major satellite builder) didn't know how to
use their nosie figure meter!

Jim
N8EE

"Steve Kavanagh" wrote in message
om...
I've been playing with trying to make rough noise figure measurements
on the cheap and have a couple of questions:

(1) Are there any issues with the following setup and procedure for
making relative noise figure measurements (e.g. comparing two
receivers) ?

____________ __________ ________
| | | | | |
|Uncalibrated| | Step | |Receiver| Audio Out
| Noise |----|Attenuator|------| Under |-------*----o (0 dB)
| Source | | | | Test | _|_
|____________| |__________| |________| | |
| | 2.7k
|_|
|
*----o (-3 dB)
_|_
| |
| | 6.65k
|_|
_|_
///
Procedu
(a) For receiver 1 connect a high impedance AC voltmeter to the audio
output marked "0 dB". Record voltage with noise source off.
(b) Turn noise source on and measure AC voltage at "-3 dB" output.
Adjust step attenuator to get same voltage as in step (a). Record
step attenuator setting.
(c) Repeat steps (a) and (b) for receiver 2.
(d) The difference in noise figure between the two receivers is the
same as the difference in attenuator settings recorded in (b) and (c).
For example if the attenuation for receiver 1 is 10 dB and for
receiver 2 is 12 dB, then receiver two has a noise figure which is 2
dB less than that of receiver 1.

Assuming this is OK we move on to question 2:

(2) To avoid the expense of a calibrated noise source, I wonder if the
repeatability from unit to unit of simple low noise amplifier circuits
(perhaps a MAR-6 ?) is good enough to allow one to be used as a noise
figure standard, at least for fairly rough measurements at HF and VHF.
I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
Has anyone measured the NF performance of simple MMIC amps at HF & VHF
? Or looked into noise figure repeatability ?

73,
Steve VE3SMA

Steve,

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

An anecdotal story for you...

About four years ago (subsequently laid off :-( ) I was working for a
company that was making a virtual (that is, used digital signal processing)
RF/Microwave measurement system. One of our customers was a Major
communications satellite builder. They were complaining that our system was
not working right for measuring noise figure. I knew it was, of course,
since I had written the software and thoroughly tested it :-). I used two
different manual methods, one of which is exactly what you are doing (except
for using acalibrated noise source) and got the same answer as my software.
Turns out that they (the Major satellite builder) didn't know how to
use their nosie figure meter!

Jim
N8EE

"Steve Kavanagh" wrote in message
om...
I've been playing with trying to make rough noise figure measurements
on the cheap and have a couple of questions:

(1) Are there any issues with the following setup and procedure for
making relative noise figure measurements (e.g. comparing two
receivers) ?

____________ __________ ________
| | | | | |
|Uncalibrated| | Step | |Receiver| Audio Out
| Noise |----|Attenuator|------| Under |-------*----o (0 dB)
| Source | | | | Test | _|_
|____________| |__________| |________| | |
| | 2.7k
|_|
|
*----o (-3 dB)
_|_
| |
| | 6.65k
|_|
_|_
///
Procedu
(a) For receiver 1 connect a high impedance AC voltmeter to the audio
output marked "0 dB". Record voltage with noise source off.
(b) Turn noise source on and measure AC voltage at "-3 dB" output.
Adjust step attenuator to get same voltage as in step (a). Record
step attenuator setting.
(c) Repeat steps (a) and (b) for receiver 2.
(d) The difference in noise figure between the two receivers is the
same as the difference in attenuator settings recorded in (b) and (c).
For example if the attenuation for receiver 1 is 10 dB and for
receiver 2 is 12 dB, then receiver two has a noise figure which is 2
dB less than that of receiver 1.

Assuming this is OK we move on to question 2:

(2) To avoid the expense of a calibrated noise source, I wonder if the
repeatability from unit to unit of simple low noise amplifier circuits
(perhaps a MAR-6 ?) is good enough to allow one to be used as a noise
figure standard, at least for fairly rough measurements at HF and VHF.
I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
Has anyone measured the NF performance of simple MMIC amps at HF & VHF
? Or looked into noise figure repeatability ?

73,
Steve VE3SMA

"bviel" wrote in message ...

Thanks for the suggestions. I'll have a look at the web references.
Just a couple of notes on things that won't work.

Noise performance of Mar's MMIC's should be noted on datasheets.

Yes, but they never show the range of noise figures, only a typical or
worst case numbers. Of course if the noise figure is very low (PHEMT)
then the variability is pretty small as long as the circuit is done
right.

Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.

This doesn't work, because the noise is dominated by the receiver
noise, not the resistor thermal noise (unless the receiver is very
very good !).

73,
Steve VE3SMA

"Jim" wrote in message ...

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

73,
Steve VE3SMA

Steve Kavanagh wrote:
"Jim" wrote in message ...

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

You might find Terry Ritter's work on getting a good noise source to
be of at least a bit (ahem!) of interest:

http://www.ciphersbyritter.com/NEWS5/FMRNG.HTM

There are lots of other hits in Google for a search on
'calibrated "pseudo random" noise'
(without the outer single quotes).

It's hard to generate good noise, and at least as hard to find it.

--
Mike Andrews

Tired old sysadmin

"Steve Kavanagh" wrote in message
om...
"Jim" wrote in message
...

It has been a couple of years since I have done a noise figure
measurement
so my brain may be a bit rusty, but I don't see anything wrong with
this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the
'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

73,
Steve VE3SMA

Sorry about the double post of my response. I had a major problem with my
hard drive here and had to reload Windows XP from scratch. Evidently
Outlook Express (my news reader) burped the first time I used it.


Anyway....

As other's have said, there are ways to build a calibrated noise source.
I've even seen zener diodes and just plain old switching diodes used. Just
do a web search. You may even be able to find a commercial one at a flea
market, but I would question its quality.

What frequency are you using? One thing to keep in mind is that any loss or
mismatch can affect your measurement. We (when I was doing the software)
were operating at up to 32 GHz. At that frequency microscopic burs on the
sub miniature coax connectors caused all kinds of headaches!.

Jim
N8EE

"Jim" wrote in message ...

As other's have said, there are ways to build a calibrated noise source.
I've even seen zener diodes and just plain old switching diodes used.

I use a 1N21 (in reverse breakdown) as an uncalibrated source up to
about 5 GHz. A friend uses a 1N23 to 10 GHz. But how can it be
calibrated without using professional test gear ? I don't know.

What frequency are you using?

Anywhere from HF to 24 GHz is of interest ! I have used the relative
noise figure measurement scheme between 3.5 & 903 MHz so far.

One thing to keep in mind is that any loss or
mismatch can affect your measurement.

Indeed...I assume that the use of a reasonable minimum attenuation in
the step attenuator will minimize the effects of receiver input
mismatch on the measurement accuracy (assuming the attenuator is
itself well matched). I rather doubt that I would be able to homebrew
an accurate measurement system at 24 GHz (or even 10 GHz). But
construction tolerance issues should not be a problem at HF or (with
care) at VHF.

73,
Steve VE3SMA

"Steve Kavanagh" schreef in bericht
om...
"bviel" wrote in message
...

Thanks for the suggestions. I'll have a look at the web references.
Just a couple of notes on things that won't work.

Noise performance of Mar's MMIC's should be noted on datasheets.

Yes, but they never show the range of noise figures, only a typical or
worst case numbers. Of course if the noise figure is very low (PHEMT)
then the variability is pretty small as long as the circuit is done
right.
The noise figure of a MMIC is flat from DC to Ghz, the one I worked with.
Did not test the Mar's.

Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.

This doesn't work, because the noise is dominated by the receiver
noise, not the resistor thermal noise (unless the receiver is very
very good !).
The method is from JT44 EME software where you can measure noise figures.
The Help with the program should explain everything.
I myself am in the position to measure noise with my scope for a "second"
opinion.
The noise of a high frequency transistor b-e diode should deliver the right
sort of noise
also called white noise.
Or high frequency diode microwave type.
The transistor has because of his junction noise behavior the most real
white noise,
or general white noise.




73,
Steve VE3SMA

By the way....

I worked out your method of Noise Figure measurements of two receivers
at a mathematical way and it seemed right.
I am glad the same statement where made by the professionals under us.
They are a bit "rusty" but we need them !


"bviel" schreef in bericht
...

"Steve Kavanagh" schreef in bericht
om...
"bviel" wrote in message
...

Thanks for the suggestions. I'll have a look at the web references.
Just a couple of notes on things that won't work.

Noise performance of Mar's MMIC's should be noted on datasheets.

Yes, but they never show the range of noise figures, only a typical or
worst case numbers. Of course if the noise figure is very low (PHEMT)
then the variability is pretty small as long as the circuit is done
right.
The noise figure of a MMIC is flat from DC to Ghz, the one I worked with.
Did not test the Mar's.

Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.

This doesn't work, because the noise is dominated by the receiver
noise, not the resistor thermal noise (unless the receiver is very
very good !).
The method is from JT44 EME software where you can measure noise figures.
The Help with the program should explain everything.
I myself am in the position to measure noise with my scope for a "second"
opinion.
The noise of a high frequency transistor b-e diode should deliver the
right
sort of noise
also called white noise.
Or high frequency diode microwave type.
The transistor has because of his junction noise behavior the most real
white noise,
or general white noise.




73,
Steve VE3SMA

By the way....

I worked out your method of Noise Figure measurements of two receivers
at a mathematical way and it seemed right.
I am glad the same statement was made by the professionals under us.
They are a bit "rusty" but we need them !
http://www.mth.msu.edu/~maccluer/Lna/noisetemp.html
Amplifier Noise Measurements layout linked, by a German company
that sells LNA's.


"Steve Kavanagh" schreef in bericht
om...
"bviel" wrote in message
...

Some assorted comments and follow-on questions on your suggestions:

- (from bviel) The Elecraft noise generator is not a calibrated unit.
They give a typical ENR but specifically state that it varies from
unit to unit. Their low-level signal generator is calibrated (though
perhaps not quite accurately enough) but this would bring in the issue
of measurement errors between noise and sine-wave powers. And it only
works at one frequency.

- (from bviel) Which MMIC did you find had flat noise figure to 1 GHz
? My experience is that MMIC NF specs are usually at a frequency
above the flat part of the spectrum, as this is more indicative of the
high frequency performance. Are there any with noise figure also
specified, or at least well characterized, at low frequencies (within
the spectral region where the NF is flat) ? But the other question
with MMICs is the unit-to-unit variation in noise figure which I don't
believe is ever specified. If all units of a given part have, say,
between 2 and 3 dB noise figure then a MMIC amp could be a fairly good
standard for amateur use, but if the variation is from 1 to 4 dB then
the usefulness is limited, in my opinion.

- (from bviel) I don't have the JT-44 software but I did look at the
(full) manual. On page 35 it describes the "Measure Sub-Mode" which
allows noise levels to be compared. However it does not appear to
support absolute noise figure measurements (unless you use a
calibrated noise source). Is there more in the online help ?

- (from Mike Andrews) Terry Ritter's stuff seems mostly concerned with
the degree of randomness rather than absolute output power, which
makes sense as he is concerned with cryptography. But I did find,
elsewhere on his web page,

http://www.ciphersbyritter.com/REALRAND/92102201.HTM

a low frequency noise generator based on a diode source followed by a
limiting amp. I imagine this concept could be extended to HF by
suitable choice of amplifier. I am not sure of the significance of
the very different output waveform (pulses of random width and fixed
amplitude) compared to the true random (Gaussian) noise from the diode
itself. I am not sure if I am willing to tackle the math needed to
understand this or to calculate the ENR - can anyone help ?

73,
Steve VE3SMA

Steve,

There is another method of measuring noise figure that does not need a noise
source.

The reason that it is not used as often is that it is harder to measure low
noise figures accurately. Since there are still a few layers of rust on my
brain I will not geive the details here, since i do not want to say anything
that is too far off.

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated properly.
The noise output is a combination of the DUT noise and the 'ambient' noise
from the input termination, times the gain of the receiver. Working
backwards you can then determine the DUT noise. As you can probably guess,
the reason it doesn't work so good on small noise figures is that any errors
in the measurement can easily outnumber the actual noise figure you are
trying to measure! And I have actually had to deal with this problem. That
is where half my gray hairs came from (the other half from being laid off).

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Jim
N8EE

Can you measure the noise power of the noise source by comparing
the output (through the RX) with the power from a signal
generator (again through the RX) of known output power?
Presumably using some form of averaging of multiple readings
across the RX passband.

- Stewart G3YSX

Steve Kavanagh wrote:

Some assorted comments and follow-on questions on your suggestions:

- (from bviel) The Elecraft noise generator is not a calibrated unit.
They give a typical ENR but specifically state that it varies from
unit to unit. Their low-level signal generator is calibrated (though
perhaps not quite accurately enough) but this would bring in the issue
of measurement errors between noise and sine-wave powers. And it only
works at one frequency.

- (from bviel) Which MMIC did you find had flat noise figure to 1 GHz
? My experience is that MMIC NF specs are usually at a frequency
above the flat part of the spectrum, as this is more indicative of the
high frequency performance. Are there any with noise figure also
specified, or at least well characterized, at low frequencies (within
the spectral region where the NF is flat) ? But the other question
with MMICs is the unit-to-unit variation in noise figure which I don't
believe is ever specified. If all units of a given part have, say,
between 2 and 3 dB noise figure then a MMIC amp could be a fairly good
standard for amateur use, but if the variation is from 1 to 4 dB then
the usefulness is limited, in my opinion.

- (from bviel) I don't have the JT-44 software but I did look at the
(full) manual. On page 35 it describes the "Measure Sub-Mode" which
allows noise levels to be compared. However it does not appear to
support absolute noise figure measurements (unless you use a
calibrated noise source). Is there more in the online help ?

- (from Mike Andrews) Terry Ritter's stuff seems mostly concerned with
the degree of randomness rather than absolute output power, which
makes sense as he is concerned with cryptography. But I did find,
elsewhere on his web page,

http://www.ciphersbyritter.com/REALRAND/92102201.HTM

a low frequency noise generator based on a diode source followed by a
limiting amp. I imagine this concept could be extended to HF by
suitable choice of amplifier. I am not sure of the significance of
the very different output waveform (pulses of random width and fixed
amplitude) compared to the true random (Gaussian) noise from the diode
itself. I am not sure if I am willing to tackle the math needed to
understand this or to calculate the ENR - can anyone help ?

73,
Steve VE3SMA

"Jim" wrote in message ...

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated
properly

....any errors
in the measurement can easily outnumber the actual noise figure

Thanks for reminding me of that one, Jim. But I see your point about
errors. For example, in measuring the gain one needs a standard. One
of the few pieces of real test gear I have is a bolometer-type RF
power meter which can measure about -13 dBm accurately. If the
measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
thermal noise is -114 dBm. So I need about 100 dB gain for a very low
noise figure DUT. To measure that I might need five 20 dB attenuators
as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
there's +/-2.5 dB error (well, I suppose I could cross my fingers and
RSS the numbers). Or I have a diode-type power meter that will
measure lower power, which leads to issues of how the detector
responds to noise. And then there's the problem of knowing the noise
bandwidth precisely...

is where half my gray hairs came from (the other half from being laid off).

I think about half of mine come from the latter factor too !

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

Definitely. Though, with my method the input never gets more than 3
dB above the receiver noise floor and in most cases a well designed
receiver will have no AGC response at that level. But with a preamp
in front it usually will activate the AGC, so AGC has to be switched
off when comparing preamps - which of course is impossible to do in
most ham rigs !

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Hence the interest in NF repeatability of MMIC amps, since they are
hard to build wrong, are well matched over a wide bandwidth and don't
require tuning for best noise figure. The known DUT can also be the
standard itself (in association with a receiver of only roughly known
NF) to avoid issues of errors in calibrating the noise source ENR.

73,
Steve VE3SMA

You also have to know the noise bandwidth of the system to use this
method. If the response is dominated by a single, fairly steep-sided
filter, this is easy. But otherwise (such as if both IF filtering and AF
amplifier response shape the overall response), some calculation and/or
measurement is required. I've gotten what I believe are reasonable
results on HF and AF amplifiers using this method. But I haven't tried
it on a really low NF system, so would heed Jim's caution.

Roy Lewallen, W7EL

Jim wrote:

Steve,

There is another method of measuring noise figure that does not need a noise
source.

The reason that it is not used as often is that it is harder to measure low
noise figures accurately. Since there are still a few layers of rust on my
brain I will not geive the details here, since i do not want to say anything
that is too far off.

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated properly.
The noise output is a combination of the DUT noise and the 'ambient' noise
from the input termination, times the gain of the receiver. Working
backwards you can then determine the DUT noise. As you can probably guess,
the reason it doesn't work so good on small noise figures is that any errors
in the measurement can easily outnumber the actual noise figure you are
trying to measure! And I have actually had to deal with this problem. That
is where half my gray hairs came from (the other half from being laid off).

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Jim
N8EE

On 14 Oct 2004 16:44:35 -0700, (Steve
Kavanagh) wrote:

I haven't really been following this thread but if you are looking for
noise diodes and associated stuff, you should take a look at:

http://www.noisecom.com/

Seems to me that at one time they would calibrate a homebrew source
for a modest fee. That may have been a ham that worked there paving
the way; not sure.




|"Jim" wrote in message ...
|
| This other method involves measuring the gain of the device under test and
| then measuring the noise power output with the input terminated
| properly
|
| ....any errors
| in the measurement can easily outnumber the actual noise figure
|
|Thanks for reminding me of that one, Jim. But I see your point about
|errors. For example, in measuring the gain one needs a standard. One
|of the few pieces of real test gear I have is a bolometer-type RF
|power meter which can measure about -13 dBm accurately. If the
|measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
|thermal noise is -114 dBm. So I need about 100 dB gain for a very low
|noise figure DUT. To measure that I might need five 20 dB attenuators
|as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
|there's +/-2.5 dB error (well, I suppose I could cross my fingers and
|RSS the numbers). Or I have a diode-type power meter that will
|measure lower power, which leads to issues of how the detector
|responds to noise. And then there's the problem of knowing the noise
|bandwidth precisely...
|
| is where half my gray hairs came from (the other half from being laid off).
|
|I think about half of mine come from the latter factor too !
|
| If you are measuring an entire receiver there are a few things you have to
| be careful with. The receiver must be a linear receiver (no FM, AM diode
| detector, etc.---basically just SSB). There should be a filter to pick just
| one sideband. Turn the AGC off. Make sure you measure the gain in the
| linear region, which also applies to a simple amplifier.
|
|Definitely. Though, with my method the input never gets more than 3
|dB above the receiver noise floor and in most cases a well designed
|receiver will have no AGC response at that level. But with a preamp
|in front it usually will activate the AGC, so AGC has to be switched
|off when comparing preamps - which of course is impossible to do in
|most ham rigs !
|
| If you have a DUT with a known noise figure, I think that this would be one
| way of calibrating a homebrew noise source.
|
|Hence the interest in NF repeatability of MMIC amps, since they are
|hard to build wrong, are well matched over a wide bandwidth and don't
|require tuning for best noise figure. The known DUT can also be the
|standard itself (in association with a receiver of only roughly known
|NF) to avoid issues of errors in calibrating the noise source ENR.
|
|73,
|Steve VE3SMA

I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
If you are satisfied with +/- 1 to 2 dB variation, the link I gave to
the Mar MMIC 35dB "calibrated" Noise generator should be
accurate enough.
That why I "forgot" the approximatly statement.

You can not compare noise power with a calibrated sine wave generator.
That's right, not directly.
You can calculate the RMS for a sine wave.
Also calculate the RMS for noise power, but its another formule.
Use the same impedances and frequency's.


If you know the amount of (milli) Ampere's through the noise diode and
know the impedance of the load, you can make power calculations like
we do with DC.
With the calculation method you don't need to have a calibrated noise head.
Someone else discussed the method, let me give the formule.
The formule is in Chris Bowick's book RF Circuit Design.
Its about Shot Noise. (Not thermal)
In^2 = 2qIdcB

In^2 = the mean square noise current
q = the electron charge (1.6 x 10e-19 coulombs)
Idc = the direct current in Ampere's
B = the bandwith in Hertz

Onces you have the mean square current, calculate the power in the load
resistor.
Did not made calculations with it, since I have the opportunity to measure
noise
right away. Will do it in future to check the formule.

The MMIC I used.... I was afraid there would come a question about it.
It's in a factory designed preamp of Japanese origin, has a forgotten
product
number, I mean National, not sure of that.
Searched hours for datasheets, because I was convinced the NF of the device
was less at 144Mhz, the manual stated approximatly 2dB 2Ghz.
EME use 144Mhz, with the knowledge of bipolar transistors that have an
increase of NF with frequency, 144Mhz would have a NF of about 1 - 1.5dB.
The manual said nothing about a lower NF at lower frequency's.
So bought it with the deal, not good money back.
Found the datasheets, the NF was 2.8dB flat from almost DC to 2Ghz.
From there comes the idea of flat NF respons of MMIC's.
I brought the device back to store and got money back. 150$.
More research learnt me that similar devices with less NF in order of 0.7dB
were avaiable for 20$, OK no box around it, no blinky leds.
I know only that the MMIC is obsolete today, forgot the type number.

A single mosfet, BF981 does 1.7dB at 100Mhz and cost about 1$.
I love homebrewing.
With that device you could also calibrate your noise generator.
It matched the input impedance of the 50 Ohm receiver.
Many EME amateurs use it in their preamps.
If they all use the same scheme it would be accurate within a variation
of say +/- 0.1dB ?

Since I can measure noise power, how to calibrate whitout a calibrated
noise head is not a hot item for me anymore.
Just practice.
Before that, I read several books, spent hours on the internet, collected
many
data.
Its only to share thoughts, nothing science, professional or years of
experience,
and yes I can be wrong.
The goal was maybe you can do something with it, for me, maybe there is a
clever
way to determine the NF without calibrated noise head, maybe even whitout
math.

The JT44 software program, I just finished the interface from receiver to
PC.
Details at the original PSK31 homepage.
Used a "Jensen" audio trafo, to prevent ground loops.
A lot to do... no practical data at this moment.

Have read Terry Ritter's stuff.
He said, no white noise at all. Just pink. See the graph's.
Read in some university books, noise is of pink behavior at the lowest
frequency's.
The graph showed the audio band.
White noise is flat (whitin 15 - 20Khz partitions) and at higher
frequency's, and random.
Whit respect to the measurements of course.

There is an increase of NF to higher frequency regions.
How came that MMIC to a flat respons ?

Sorry I was wrong about the noise figure at 100Mhz for the mosfet BF981.
It has to be 0.7dB instead of 1.7dB!

It can still better with the BF998, to get an impress of the scheme try the
following link http://lea.hamradio.si/~s53ww/4xbf998/4xbf998.htm

I think it can be reproducible whitin small variations.

Wes Stewart wrote:

I haven't really been following this thread but if you are looking for
noise diodes and associated stuff, you should take a look at:

http://www.noisecom.com/

Seems to me that at one time they would calibrate a homebrew source for
a modest fee. That may have been a ham that worked there paving the
way; not sure.

In connection with the QST article by Bill Sabin, W0IYH, NoiseCom had a
program of single-quantity sales to amateurs, and offered a low-cost
calibration service. I'm not sure whether that program still operates,
but some years ago NoiseCom were very helpful in supplying a free sample
outside of the USA.

If I were building a noise source again, I wouldn't put NoiseCom to any
trouble. Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

ENR flatness of the whole noise source depends on your construction. The
other important factor is that the output impedance must not change
significantly between the 'on' and 'off' states of the diode. To achieve
both of these, the best technique is to build the noise source using
SMD/microstrip technique including an attenuator of about 7dB made from
0805 SMD resistors; and then buy a high-class 20dB attenuator (N or SMA)
which need not be expensive at a hamfest. This attenuator then becomes a
permanent part of the noise source - not to be taken off and used for
something else!

This level of attenuation will give you an ENR of about 5-6dB, which is
what you need to measure typical modern low-noise amplifiers.

On the DC side, it is a very good idea to include a constant current
source for long-term stability of ENR. Most devices will give a peak of
ENR at a few mA, so you need to adjust the current to the top of this
peak where the variability is least. (There is much more noise at
currents of a few hundred uA, but you don't want to go there - the noise
output there is far too sensitive to the DC current, temperature, color
of carpet, phase of moon etc.)

It is VERY important to design for the industry-standard power supply of
+28V DC, so that your noise source is a simple plug-in replacement for
any professional source... because that is how you're going to get your
ENR calibration.

All of these ideas come from an article by DJ9BV in DUBUS magazine which
described a very high-class noise source, good up to 10GHz. Mine uses
simpler and much less precise construction, so the ENR begins to wobble
above a few GHz due to resonances.

The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

As for calibration, the best way is to take your noise head to a
microwave meet where there is professional NF measuring equipment, and
take a cal from the HP346A there. Alternatively, get to know someone -
anywhere - who has access to these facilities, and can do it for you one
lunchtime.

Other useful background information is in an Agilent App Note, at:
http://literature.agilent.com/litweb/pdf/5952-3706E.pdf

If you can still find a copy of 'The VHF/UHF/DX Book' (out of print)
there's a lot of information in there too.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK" wrote in message ...

Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

That's an interesting suggestion...but presumably the 35 dB ENR can't
be counted upon to the level needed for measurements without
calibration.

The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

Thanks, Ian. I found the DUBUS articles...though I haven't read them
through yet. They look very interesting. Though perhaps not *exactly*
what I was after, as they use a real microwave noise diode, whereas my
thrust is more in the direction of what can be done with more common
parts and without any need for calibration of individual noise
standards. I recognize that this probably won't be possible in the
microwave range but it seems likely (to me, anyway) that reasonable
accuracy (not good enough for EME preamps !) can probably be attained
up to VHF, even with these constraints.

73,
Steve, VE3SMA

Steve Kavanagh wrote:
"Ian White, G3SEK" wrote in message
...

Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

That's an interesting suggestion...but presumably the 35 dB ENR can't
be counted upon to the level needed for measurements without
calibration.

That's correct; it is only a design guide to the amount of attenuation
that will be needed to give an ENR that's in the right ballpark. But
then you need to know what the actual value *is* - and for that, you
still need a calibration.


The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

Thanks, Ian. I found the DUBUS articles...though I haven't read them
through yet. They look very interesting. Though perhaps not *exactly*
what I was after, as they use a real microwave noise diode, whereas my
thrust is more in the direction of what can be done with more common
parts and without any need for calibration of individual noise
standards.

My point was that you don't need a real microwave noise diode - any
small, cheap UHF/microwave bipolar transistor will give almost the same
performance up to several GHz.

I recognize that this probably won't be possible in the
microwave range but it seems likely (to me, anyway) that reasonable
accuracy (not good enough for EME preamps !) can probably be attained
up to VHF, even with these constraints.

You can optimize NF using an uncalibrated source, but with this kind of
source you cannot can make worthwhile quantitative measurements without
an external calibration.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Stewart Bryant" wrote in message
news:1097795826.35817@sj-nntpcache-3...
Can you measure the noise power of the noise source by comparing
the output (through the RX) with the power from a signal
generator (again through the RX) of known output power?
Presumably using some form of averaging of multiple readings
across the RX passband.

- Stewart G3YSX

Steve Kavanagh wrote:

Some assorted comments and follow-on questions on your suggestions:

- (from bviel) The Elecraft noise generator is not a calibrated unit.
They give a typical ENR but specifically state that it varies from
unit to unit. Their low-level signal generator is calibrated (though
perhaps not quite accurately enough) but this would bring in the issue
of measurement errors between noise and sine-wave powers. And it only
works at one frequency.

- (from bviel) Which MMIC did you find had flat noise figure to 1 GHz
? My experience is that MMIC NF specs are usually at a frequency
above the flat part of the spectrum, as this is more indicative of the
high frequency performance. Are there any with noise figure also
specified, or at least well characterized, at low frequencies (within
the spectral region where the NF is flat) ? But the other question
with MMICs is the unit-to-unit variation in noise figure which I don't
believe is ever specified. If all units of a given part have, say,
between 2 and 3 dB noise figure then a MMIC amp could be a fairly good
standard for amateur use, but if the variation is from 1 to 4 dB then
the usefulness is limited, in my opinion.

- (from bviel) I don't have the JT-44 software but I did look at the
(full) manual. On page 35 it describes the "Measure Sub-Mode" which
allows noise levels to be compared. However it does not appear to
support absolute noise figure measurements (unless you use a
calibrated noise source). Is there more in the online help ?

- (from Mike Andrews) Terry Ritter's stuff seems mostly concerned with
the degree of randomness rather than absolute output power, which
makes sense as he is concerned with cryptography. But I did find,
elsewhere on his web page,

http://www.ciphersbyritter.com/REALRAND/92102201.HTM

a low frequency noise generator based on a diode source followed by a
limiting amp. I imagine this concept could be extended to HF by
suitable choice of amplifier. I am not sure of the significance of
the very different output waveform (pulses of random width and fixed
amplitude) compared to the true random (Gaussian) noise from the diode
itself. I am not sure if I am willing to tackle the math needed to
understand this or to calculate the ENR - can anyone help ?

73,
Steve VE3SMA


The answer is: Sort of.

Another post I made mentions that if you know the gain of your DUT you can
calculate the noise figure by measuring the noise power output with the
input terminated properly. Likewise, I think you could determine the ENR of
a noise source by the same method.

Someone else mentioned calculating the noise power of the ENR from the
current used. This would give the total noise power, but what is needed is
the noise power at a very narrow band of frequencies. If you are trying to
measure a receiver, the receiver determines the bandwidth, but with an LNA
you need a narrow band detector.

When I was doing the afore mentioned software, I used a FFT power spectrum
and made four measurements (DUT with noise source, DUT with out noise
source, test receiver with noise source, test receiver without noise
source). The FFT provided the narrow band filtering.

Of course, watch out for double sideband fold over in the mixer!

Jim
N8EE

Mike Andrews wrote:

[...]

You might find Terry Ritter's work on getting a good noise source to
be of at least a bit (ahem!) of interest:

http://www.ciphersbyritter.com/NEWS5/FMRNG.HTM

There are lots of other hits in Google for a search on
'calibrated "pseudo random" noise'
(without the outer single quotes).

It's hard to generate good noise, and at least as hard to find it.

--
Mike Andrews

Tired old sysadmin

Also take a look at Terry's analysis of various other noise sources:

http://www.ciphersbyritter.com/NOISE/NOISRC.HTM

Good noise is hard to find:)

Mike Monett

I read Terry's work again, the comments and links.

White noise generated with the B-E junction of a high
Ft transistor, B-E junction is a zener.
If you downconvert above 100kHz, that's where white
noise starts, and display it with your soundcard you
"should" get a flat respons.
The methods where right, the spectrum used was too
low.
Add zener noise to your soundcard and you get pink
noise, because at low frequenties the noise behavior
is always pink + the 1/f semiconductor flicker noise.
You can justify it, but that's manmade white noise.
If I was interested in random numbers, I would use
real white noise, real random.
The keyword here is downconvert in KISS concept,
simple as possible.
Semiconductors produce also white noise.
If you can't display white noise, that does not mean
the noise generator produces non.
I would not trust my sound card at all, because with no
input the FFT shows pink noise, ok,at very low level,
but its added to your not anymore random signal.
I have tried Spectran FFT software.
Peaks from noise floor up to 20dB in the range of zero
to hundred hertz.
The computer is full of signals inside, that's not random
compared to white noise.
If you are measuring relative great signals, the little noise
does not improve the S/N ratio so much that you cannot
copy the signal anymore.
But random noise added with little pink noise is no longer
random, how small the error is, especially if the error is only
at a specifiek part of the spectum.
Maybe a professional A/D chip can do the job ?
Unfortunate a spectrum analyzer can show you the white noise,
but cannot make numbers out of it.
To make numbers out of it is the difficulty I think, not to get
white noise.
The analog world and digital world have a " love and hate"
relationship, sometimes they work fine together,
sometimes not.
I don't say the above statements are right, its just how I think
about the experiments at my point of view.